Methods for the separation and enrichment of mixtures of isotopes or structural isomers typically utilize distillation techniques. Mixtures of isotopes and structural isomers and the like often have separation factors close to 1. For example, the separation factor for xylene isomers (ratio of para/ortho vapor pressures) at a pressure close to atmospheric pressure is approximately 1.2, and the separation factor for the oxygen isotope 18O (ratio of 16O16O/16O18O vapor pressures) is approximately 1.01.
In the case of this type of mixture, a packed column system is normally used for the distillation column. However, in the case of a distillation of a mixture with a separation factor extremely close to 1, in order to ensure the necessary vapor liquid contact region required for separation, either the number of distillation column must be increased, or the total length of a single column must be made as long as possible. In other words, in a distillation separation method, because the vapor liquid contact section of the distilling column lengthens, the quantity of liquid holdup of the targeted components within the distilling column increases. Consequently, in this type of distillation apparatus, the start-up time (the time from the commencement of operations until a product is obtained) will be long. (Normally, vapors have much smaller densities than liquids, meaning vapor holdup can be ignored.)
For example, Dostrovsky et al. have reported that the start-up time for a distillation apparatus for separating the oxygen isotope 18O using water is at least 480 days (I. Dostrovsky and M. Epstein: “The Production of Stable Isotopes of Oxygen”, Stable Isotopes pp. 693-702 (1982) Elsevier Scientific Publishing Company, Amsterdam).
In this type of distillation apparatus, reducing the quantity of liquid holdup within the distillation apparatus can be a direct method of shortening the start-up time. Accordingly, in conventional distillation separation of isotopes and the like, actions such as replacing random packing with structured packing, or reducing dead volume from the reboiler without impeding the liquid flow, have been performed. Examples include installing embedded structures within the liquid accumulated in the reboiler which do not impede the liquid flow, and a method without liquid pumps which connects a plurality of distillation columns in a cascade arrangement (an example of this method is disclosed in Unpublished Japanese Patent Application, No. Hei 11-290259: “Apparatus, method for enrichment of the heavy isotopes of oxygen and production method for heavy oxygen water”).
A schematic diagram of a conventional example of a method of separating and enriching a mixture with a separation factor close to 1, such as the separation of isotopes or structural isomers, is shown in FIG. 7. Using oxygen isotopes as an example of a mixture with a separation factor close to 1, FIG. 7 shows the flow diagram for enriching the low volatile oxygen isotopes 17O and 18O through oxygen distillation. In this conventional example, in order to allow any start-up time reduction effects to be observed clearly, the case of a distillation apparatus cascade with 10 distillation columns is investigated.
A “cascade” refers to connecting distillation columns so that the enriched product by the one distillation column is further enriched by the next column, and then even further enriched by one after another, and the overall combination of distillation columns constructed in this manner is termed a “cascade process”.
In FIG. 7, the feed is ultra high purity oxygen (flow rate 2.816 mol/s) and incorporates the oxygen isotopes in their natural abundance ratios (refer to Table 1 and Table 2).
TABLE 1ElementAbundance ratio16O0.9975917O0.0003718O0.00204Total1.00000
TABLE 2Mass numberOxygen moleculeAbundance ratio3216O16O0.995193316O17O0.000743416O18O0.004073417O17O1.37E−073517O18O1.51E−063618O18O4.16E−06Total1.00000
Each distillation column filled with a structured packing with a specific surface area of 500 m2/m3 and a packing height of 45 m, and is operated at an overhead pressure of 1.2 bar (absolute pressure) and a superficial vapor load Fs (F-factor) of approximately 1.6. The 10 distillation columns are connected in a cascade, and first, ultra high purity oxygen is introduced as the feed into a first column 8 from a pipeline 1. The isotopes of the introduced ultra high purity oxygen are separated into a low volatile component and a high volatile component by distillation, and the high volatile component exits from the top of the first column 8 as an exhaust gas, and passes through a pipeline 2, and then through a condenser 9 and out of a pipeline 3. A portion of this high volatile component is returned to the upper section of the first column 8 via a pipeline 4.
In contrast, at the bottom of the first column 8, a bottom liquid with an enriched low volatile component passes into a pipeline 5, and following vaporization in a reboiler 10, a portion is fed through a pipeline 7 into the upper section of a second column 11, while the remainder passes through a pipeline 6, which is split from the pipeline 7, and introduced into the first column 8, where it becomes the ascending vapor of the first column 8. The low volatile components fed into the upper section of the second column 11 via the pipeline 7 undergoes further enrichment of the low volatile component in the second column 11.
The low volatile component enriched in the second column 11 is fed to a condenser 12, and the high volatile component is fed into a reboiler 13.
By repeating this process, eventually the enriched low volatile component is fed into the upper section of a tenth column 34 from a pipeline 30, and the further enriched low volatile component pulls out the bottom of the tenth column 34 via a pipeline 31, and the enriched low volatile vapor produced by vaporization in a reboiler 36, passes through a pipeline 35, and is delivered out of a pipeline 33 as a final product. The ascending vapor required in the tenth column 34 is supplied to the lower section of the tenth column 34 from a pipeline 32 split off from the pipeline 35. This cascade system is described in detail in Unpublished Japanese Patent Application, No. Hei 11-290259, and in comparison with the cases in which liquid pumps are used, is able to reduce the quantity of liquid holdup and the amount of heat-inleak.
The heat-exchange-fluid used in the heat exchangers (condenser and reboiler) attached to each column may utilize any of nitrogen, oxygen, air, or the exhaust gas from an air separation unit, and usually a circulation (not shown in the figure) is formed connecting the condenser and the reboiler, enabling continuous operation. The concentration distributions within each column in this conventional example are shown in FIG. 8. In FIG. 8, the packing height shown on the horizontal axis is defined so that the top of the first column 8 is set as a packing height of zero, and the packing heights of the subsequent columns are then added on, so that the bottom of the tenth column 34 corresponds with a packing height of 450 m. A portion of the vapor from the reboiler 36 of the tenth column 34 is produced via the pipeline 33 as the product (flow rate 0.0352 mol/s). As a result of the ten column cascade, 16O17O is enriched from 0.074% to 2%, and 16O18O is enriched from 0.407% to 25%. The recovery rates of 17O and 18O are 34.3% and 76.6% respectively. The production rate, when converted to water, amounts to 40 tones of 17O water (17O 1 atom %) per year, which represents an industrial scale production for the isotope.
However, in terms of the distillation method and apparatus, dephlegmators (combined heat exchange and distillation apparatuses) are already known. The main purpose of using these dephlegmators is to energy-saving and enable a more compact apparatus. Examples of energy-saving are disclosed in Unexamined Japanese Patent Application, First Publication No. Hei 8-66601 and Unexamined Japanese Patent Application, First Publication No. Hei 8-131704 (benzene-toluene, HiDiC). Furthermore, an example of a more compact apparatus is disclosed in Unexamined Japanese Patent Application, First Publication No. Hei 11-153383 (Method and device for producing nitrogen). In addition, an example targeting both energy-saving and a more compact apparatus is disclosed in U.S. Pat. No. 5,921,108 (Reflux condenser cryogenic rectification system for producing lower purity oxygen).
However, no evidence has been provided as to whether or not the liquid holdup within a distillation apparatus can be reduced using these types of dephlegmators.
As described above, when a distillation method is used to separate and enrich a mixture with a separation factor close to 1, the problem of increased liquid holdup within the distillation apparatus arises.